1. Technical Field
The present invention relates to earth boring machines, and in particular to an improved bit for forming large-diameter raise holes.
2. Background Art
Vertical shafts between different levels of a mine are required to provide, for example, ventilation, access for rescue in emergency situations, and passageways for ore transfer and for water conduits and electricity lines. These shafts are commonly formed by first drilling a small diameter pilot hole downwardly from an upper mine level to a lower level with a small diameter pilot bit attached to the lower end of a drill string composed of sections of drill pipe. The drill string is rotated about its longitudinal axis by a drilling rig anchored to the floor at the upper mine level. Sections of drill pipe are added as the pilot bit progresses downwardly. Once the desired depth of the shaft is reached, the small diameter pilot bit is removed and replaced with a much larger diameter raise bit. The raise bit is rotated and simultaneously pulled upwardly along the pilot hole by the drilling rig through the intermediacy of the drill string to enlarge the pilot hole. As the raise bit is drawn upwardly, sections of the drill pipe are progressively removed.
Typically, the raise bit comprises a cutter carrier frame rigidly fixed to a drill stem which in turn is detachably connectible to the lower end of a drill string. A plurality of roller cutters are mounted on the upper surface of the cutter carrier frame to disintegrate the earth formations surrounding the pilot hole.
A major disadvantage of existing raise bits, especially those used to form larger diameter shafts, is that a large number of cutters are needed relative to the size of the shaft formed. This stems at least in part from the fact that raise bit cutters are normally positioned on a cutter carrier frame so that their cutting edges form a domeshaped profile, with the larger the hole, the deeper the profile. This type of profile tends to laterally stabilize the raise bit so that it tracks the pilot hole. For a given diameter raise hole, however, the actual surface area swept by the cutters is larger for a deeper dome than for a more shallow one, and so the number of required cutters is larger for a deep dome. The additional cutters not only increase the initial expense of the raise bit, but also add to the cost of periodically dressing the cutters.
One problem caused by reducing the number of cutters on a bit designed to form a given size raise hole is that fewer cutters are available to prevent tilting of the bit, and high bending stresses are induced on the drill stem when a particular bit encounters a piece of especially hard or fracture resistant rock. Because the hard rock resists penetration by the cutter, the bit tends to tilt downwardly at that location. Reducing the number of cutters on the bit will result in fewer cutters being disposed on the opposite side of the bit to react against the working face of the raise hole to prevent tilting of the bit. Moreover, arranging the cutters in a rather shallow profile reduces the lateral stability of the raise bit also causing the bit to rock or tilt as it advances and thus less satisfactorily track the pilot hole.
Another problem with existing raise bits is that the cutters wear out at uneven rates. As soon as a cutter wears out, the cutterhead must be lowered to the bottom of the raise hole so that the cutter can be replaced, thus involving a time consuming, expensive operation.
During raise drilling operations, rock fractures sporadically under the tremendous loads imposed on the rock by the cutters. The sporadic fracturing of the rock in turn imparts constantly varying, resisting loads on the cutters. Thus, even though the drilling rig rotates the upper portion of the drill string at a uniform speed, the distally located raise bit usually rotates in a jerky, uneven manner, constantly accelerating and decelerating the raise bit and thereby causing high, cyclical stress peaks in the drill string possibly leading to failure of the string from fatigue. The jerky motion of the raise bit also not only reduces the efficiency of the cutters, but also causes abnormal cutter wear. The irratic motions of the raise bit is aggravated by the fact that the rotational inertia of the raise bit is very low relative to the loads imposed on the bit by the hard rock because of the typically slow speeds at which the bit is rotated, e.g. 10 to 20 revolutions per minute.
When the cutters abruptly encounter an extremely hard rock formation, the raise bit may actually stop rotating for a time while the torsionally elastic drill string "winds up" under the constant rotation of the drilling rig. It is not uncommon for the upper end portion of a drill string to rotate from 40 to 50 degrees beyond the angular position of the raise bit before enough torque is developed in the stem to overcome the resisting torque load applied to the bit by the rock formation. When the raise bit finally works free, all of the torsional energy stored in the long drill string is suddenly released thereby greatly accelerating or "whipping" the raise bit. Consequently, the raise bit overruns the drill stem thus winding up the drill string in the direction in advance of the rotation of the drilling rig. Because the rotational inertia of the raise bit has been depleted in winding up the drill stem in the advance direction, the torque available at the bit is actually less than normal, thus facilitating another jamming of the bit, thereby perpetuating the typically jerky rotational movement of the bit.
Furthermore, if sufficient windup of the drill string occurs, the raise bit may overrun the drill stem to such an extreme extent that the threaded drill string segments actually unscrew and can come apart. If this occurs, the raise bit and the portion of the drill stem still attached thereto will fall to the bottom of the shaft and be damaged, and probably beyond repair.
Known raise bit designs have not satisfactorily solved the reaming problems to which the invention is addressed. U.S. Pat. No. 4,142,593 discloses a raise drill bit incorporating an elastomeric element between the drive stem and the bit body on which the roller cutters are mounted. Although the elastomeric element may absorb some of the impact loads passing through the drill bit, it also permits the bit body to wind up relative to the drive stem thereby further aggravating the drill string windup problems discussed above.
To theoretically reduce the torque load required to enlarge a pilot hole, U.S. Pat. No. 4,076,087 utilizes a raise head having a smaller diameter, inner set of cutters which initially cuts an intermediate size raise hole and a trailing, outer set of cutters which forms the final diameter of the raise hole once the intermediate diameter raise has been formed by the inner cutters. The inner and outer set of cutters are adapted to shift relative to each other along the length of the drive stem so that they can form the raise hole in a stepwise manner. It can be appreciated that this stepwise reaming procedure, while perhaps requiring less maximum rotational torque to form a given diameter raise hole, is quite slow in operation since it requires two separate cutting operations to form a raise hole of a given diameter.
Another type of known, relevant drilling apparatus is disclosed by U.S. Pat. No. 1,391,626 wherein a flywheel is coupled to a drive shaft which connects an electric motor to a rotating cutterhead. Also, U.S. Pat. No. 3,357,744 discloses a coal planing device utilizing a flywheel to impart vibratory ramming movement to a series of chisel-like cutting tools.